1. Field of the Invention
The present invention relates to a node device installed in a network in which a plurality of node devices are connected by transmission lines, and more particularly to a node device which is used to connect a plurality of networks.
2. Description of the Related Art
Ring networks in which a plurality of node devices (hereafter referred to simply as “nodes”) are connected in ring form by transmission lines such as optical fibers or the like can be divided into the two main categories of uni-directional path switched ring (UPSR) systems and bi-directional line switched ring (BLSR) systems.
FIG. 20A is a block diagram which shows the ring network construction in a UPSR system, and the manner of data transmission in such a system. FIG. 20B is a block diagram which shows the construction of the ring network in a BLSR system, and the manner of data transmission in such a system. Both diagrams show how data that is input into the node A is output from the node D.
In the UPSR system, data that is input into the node A is transmitted to both of the nodes B and C, and is sent to the node D from the nodes B and C. In the case of normal operation, the node D selects the data from the side of the node C among the data sent from the nodes B and C, and outputs this data. The data is similarly transmitted in cases where data input into the node D is output from the node A.
Meanwhile, in the BLSR system, data that is input into the node A is transmitted only to the node C, and is then output from the node D after being sent from the node C to the node D. The data is similarly transmitted in cases where data input into the node D is output from the node D. The transmission line from the node A to the node B, and the transmission line from the node B to the node D, are utilized as bypass lines (spare channels or protection channels) for use in the case of trouble, as will be described later.
FIG. 21A shows the data transmission route used in cases where trouble occurs in the transmission path in a ring network based on the UPSR system. FIG. 21B shows the data transmission route used in cases where trouble occurs in the transmission line in a ring network based on the BLSR system. In these figures, a case is shown in which trouble has occurred in the transmission line between the node C and the node D.
In the UPSR system, even under conditions in which there is no trouble in the transmission line, data that is input into the node A is sent to the node D from the node A via the node C, and is also sent to the node D from the node A via the node B. Accordingly, in cases where trouble occurs in the transmission line between the nodes C and D, data transmitted via the route of nodes A-B-D, in which no trouble has occurred, is selected by the node D. Specifically, the node D performs processing that switches data from the side of the node C to data from the side of the node D.
On the other hand, in the BLSR system, relief measures are taken by means of an APS (automatic protection switch) protocol. Specifically, data sent to the node C from the node A is bridged by the node C and returned to the node A, after which this data is sent to the node D via the node B, switched by the node D and output from the node D.
For example, if the channels used in cases where no trouble occurs (work channels) are set as channels 1 through 24, and the channels used in cases where trouble occurs (spare channels or protection channels) are set as channels 25 through 48, then data that is transmitted from the node A to the node C using the work channel 1 is output (bridged) to both the channel 1 and the spare channel 25 by the node C. The data output to the channel 1 is not transmitted to the node D because of trouble between the nodes C and D. The data output to the channel 25 is transmitted via the nodes C-A-B-D, and in the node D, this data is returned (switched) from the channel 25 to the channel 1 and output.
In cases where a larger-scale ring network is to be constructed using a ring network of the type described above, a plurality of ring networks are connected. FIG. 22 is a block diagram which shows the construction of a network system in which four ring networks L1 through L4 are connected.
A construction which has redundancy is employed in the connection parts between the ring networks, so that even in cases where trouble occurs in the transmission lines, this can be handled by means of bypass lines. For example, in cases where trouble occurs in the transmission line between the nodes D and I, a route from the node D to the node I is formed via the nodes E and H.
However, in such conventional network interconnection parts, at least four nodes are required, so that the amount of hardware is increased, and the cost is increased.
Furthermore, in transmission lines of parts surrounded by ring networks, data (traffic) that is sent from the nodes of certain ring networks to the nodes of other ring networks is transmitted; accordingly, a larger amount of data is transmitted than in transmission lines that are not surrounded.
However, in the case of conventional ring networks, it is a prerequisite that the respective transmission lines between nodes have the same bandwidth; accordingly, convergences occur that result in bottlenecks. On the other hand, if the overall bandwidth of the ring network transmission lines is increased, this leads to a state in which the bandwidth of transmission lines other than transmission lines in parts surrounded by ring networks cannot be effectively utilized.